Field of the Invention
The present invention relates to a capacitive transducer used as an ultrasonic transducer and a method of manufacturing the capacitive transducer.
Description of the Related Art
In recent years, along with development in microfabrication technology, various micro mechanical elements fabricated with micrometer-level precision are achieved. A capacitive-micromachined-ultrasonic-transducer (CMUT) is actively developed using such technology. The CMUT is an ultrasonic device that vibrates a lightweight vibration film to transmit and receive acoustic waves (hereinafter, represented by ultrasonic waves in some cases), and can be easily provided with excellent broadband characteristics even in liquid and in air. Accordingly, if the CMUT is used for a medical purpose, the CMUT can provide a highly accurate diagnosis comparing with that with a conventionally used ultrasonic device made of a piezoelectric element, and hence the CMUT attracts attention as an alternative thereto. Note that, herein, the acoustic waves include waves called sonic waves, ultrasonic waves, and photoacoustic waves. For example, the acoustic waves include photoacoustic waves that are generated inside of a subject by irradiating the inside of the subject with light (electromagnetic waves) such as visible light and infrared light.
A capacitive transducer includes a plurality of cell structures. Each cell structure includes, for example: a first electrode placed on a substrate made of Si or the like; a second electrode that is placed so as to be opposed to the first electrode; a gap formed between the first electrode and the second electrode; a vibration film that includes the second electrode and is made of a membrane formed on the gap; and a vibration film supporting part. The membrane has a structure for sealing the gap. The capacitive transducer is formed by laminating materials on the substrate made of Si or the like, according to an example manufacturing method therefor. In this method, the gap structure is formed in the following manner. A sacrifice layer material is deposited in advance in a portion to be changed to the gap, and then the sacrifice layer is removed by etching from an opening (etching hole) provided to part of the vibration film provided on the sacrifice layer. The capacitive transducer may be used in liquid such as water and oil. If such liquid invades the gap, vibration characteristics of the vibration film become lower, and hence it is necessary to seal the etching hole provided for forming the gap. According to a capacitive transducer described in Arif Sanli Ergun et al., IEEE Transactions on Ultrasonics, Vol. 52, No. 12, December 2005, 2242-2257, sealing is performed in the following manner. That is, a gap is sealed by depositing a film of silicon nitride through low-pressure-chemical-vapor-deposition (LP-CVD) in a flow path that is communicated with the gap below a vibration film from the etching hole. Due to the nature of an apparatus used for the LP-CVD, the silicon nitride film is deposited at a substantially uniform thickness from the etching hole to the gap through the flow path, and is deposited at a thickness corresponding to the thickness of the flow path, whereby the gap is sealed. Unfortunately, in this method, the silicon nitride film may be deposited even inside of the gap through the flow path, and vibration characteristics of the vibration film may be influenced.
According to a capacitive transducer described in U.S. Pat. No. 5,982,709, a gap is formed by removing a sacrifice layer from an etching hole, similarly to Arif Sanli Ergun et al., IEEE Transactions on Ultrasonics, Vol. 52, No. 12, December 2005, 2242-2257. Further, the gap is sealed by depositing a film in the etching hole through plasma-enhanced-chemical-vapor-deposition (PE-CVD). Unlike the LP-CVD, the film deposited through the PE-CVD does not invade the insides of the gap and the flow path, and the sealing film is formed so as to be deposited on the etching hole. Accordingly, in order to seal the gap, the deposited sealing film needs to be sufficiently thick for the thickness (height) of the gap.
The thickness of the sealing film for sealing the gap of the capacitive transducer needs to be about three times the thickness of the gap. Accordingly, as the thickness of the gap becomes larger, the necessary sealing film thickness also becomes larger. Meanwhile, a reduction in membrane thickness for performance enhancement is demanded, and a configuration that enables a reduction in sealing film thickness is desired.